This invention relates to a structure to cool a helicopter tail rotor gearbox.
Helicopters having a single main-lifting rotor generally require a second smaller rotor to be mounted at the tail of the aircraft. This rotor is used to provide yaw or directional control and to counteract the torque generated by the driving of the main-lifting rotor. The tail rotor is generally driven by the same engine or engines which drive the main-lifting rotor. Since these engines are located near the main-lifting rotor, various drive shafts and gearboxes are required to transmit the power from the engines to the tail rotor.
Generally, a gearbox is required whenever the drive shaft transmitting the power changes direction. One such gearbox is located adjacent to the tail rotor hub and is connected to the rotor hub drive shaft. This particular gearbox changes the direction of the power train 90.degree..
The tail rotor receives maximum use at the hover condition. During hover, a maximum amount of power is required to counteract the torque generated by the driving of the main rotor. Although this torque must be conteracted during forward flight, the fuselage of the helicopter acts as a stabilizer thus providing some anti-torque. In helicopters having a vertical stabilizer, use of the tail rotor to counteract torque is minimized during high speed forward flight.
It is necessary to cool the tail rotor gearbox in order to increase its useful life and maintain its efficiency. Maximum cooling is required at hover and low speeds when the rotor tail is receiving maximum use. It is during the hover condition that cooling is difficult to achieve. One reason for this is the hover condition itself. It is common in the aircraft field to deflect outside ambient air over the part to be cooled. However, since a helicopter in hover is not moving forward, it is difficult to use outside ambient air for this purpose.
Helicopter tail rotors have generally been mounted either on an aft pylon or a vertical stabilizer, with the gearbox almost completely exposed to the air. In both cases, one of the methods to cool the gearbox has been simply to permit air which has been placed in motion by the rotation of the tail rotor to pass over the gearbox. One reason this has been ineffective has been the fact that the gearbox is located adjacent to the center of rotation of the tail rotor, a location where there is minimum air movement.
This method has been particularly ineffective where the gearbox has been mounted within the vertical stabilizer. When mounted in this manner the air has been unable to reach the gearbox due to the tail structure, which has compounded the problem by retaining the heat generated. A partial solution has been to permit the gearbox to protrude from the surface of the vertical stabilizer. This has helped but the gearbox is still positioned in a region of minimal air flow. Another disadvantage of this approach is that the protruding gearbox adds aerodynamic drag to the aircraft. This reduces the maximum speed at which the helicopter can travel and in general increases fuel consumption.
Another solution to the problem has been to force outside ambient air over the gearbox through the use of blowers. This has been of particular value in helicopters which have their gearbox located within the helicopter vertical stabilizer. However, a disadvantage to this approach has been that the power required to operate the blower reduced the energy available to power the aircraft. This is a significant factor since hover requires maximum power.